There are two main sources of aircraft noise: the propulsion system and aerodynamic stresses.
To reduce the aerodynamic noise, barrier materials and/or absorption materials are normally used. To reduce the noise caused by vibrations induced by the propulsion system vibration isolating devices and damping materials are typically used.
A typical example of a soundproofing panel for interiors (FIG. 1) comprises, in this order from the inside out, a structural sandwich consisting of a honeycomb panel sandwiched between two layers of epoxy prepreg, a layer of damping material, a sound-absorbing layer and a barrier layer.
In this type of panel the incident pressure wave causes a flexural stress in the structural sandwich which translates into flexural stresses concentrated at the interface between the structural sandwich and the damping layer.
The attenuation of the sound pressure wave is entrusted solely to the type of damping material used, the design parameters of which consist of the weight and the damping coefficient.
Prior examples of multilayer soundproofing panels are described in US2009/0230729A1 and US2002/0070077A1.